This invention relates to the field of arc welding using a consumable electrode and more particularly to an improved apparatus and method of short circuiting arc welding two steel plates, such as two pipe sections, together by use of a cored electrode.
In the art of welding the ends of large diameter pipe, it is conventional to machine the ends of each pipe to provide an external bevel and a narrow flat land; and to bring the machined ends into axle alignment with the lands in close but usually spaced relationship to form a weld groove which includes a gap between the two ends of the pipe. Once the pipes are in position, one or more welding heads are moved around the pipe so as to effect a 360.degree. weld. The weld is usually made in several steps. First, a root pass is made where at least the inner edges or lands of the pipes are fused and the gap between the lands filled with weld metal. Thereafter, several filler passes are made wherein the space formed by the bevel is filled so that the weld metal is at least flush with the outer surface of the pipe.
Just before and during the root pass, the ends of the pipe must be in accurate alignment with one another so as to form a quality weld between the two pipe ends. The root pass is a very critical part of the welding operation. Once the root pass is completed, the alignment of the pipes is assured and the welding of the next joint down the line can be commenced. The filler passes on the previous joint can be made at a more leisurely pace. Thus, during the root pass, a 100% sound weld bead must be laid. Soundness of the weld bead means the complete fusion of both the lands clear through to the inner surface of the pipes and the complete filling of the gap between the lands with the weld metal. Depositing of the weld metal in the gap is difficult because the weld must be made by moving the weld heads around the pipe such that the welding position varies from down-hand welding, vertical up or down welding, to overhead weld as the root pass is formed around the pipe. During the root pass, the pipe sections must be maintained in alignment throughout the root pass so as to form a quality weld. Typically, the pipes are clamped together to maintain the pipe alignment during welding. In addition to weld position complications and pipe alignment concerns, the weld metal formed during the root pass should fill the gap between the pipe sections, but the weld metal should not be allowed to pass through the gap and accumulate on the interior surface of the pipe. The weld bead should form a relatively smooth surface with respect to the interior of the pipe which has very little, if any, protrusion into the interior of the pipe. Excessive protrusion of the weld bead in the pipe can: 1) create problems with apparatuses running inside the pipes to detect the soundness of the pipe system, and 2) cause unwanted fluid mixing and turbulence as the fluids are transported through the pipe system.
In order to overcome the problem of the weld bead protruding into the interior of the pipe, it is common practice to make a root pass from the interior of the pipe. Such a welding method insures that the land of the weld bead during the root pass is controlled so as to prevent unwanted protrusion into the interior of the pipeline. However, such a welding method requires specially designed and costly equipment. In addition, such a welding method is very time-consuming and cost-preventative in various types of applications. Furthermore, such a welding method can only be used for large diameter pipes. Smaller diameter pipes cannot accommodate the welding apparatus inside the pipe. Another method of welding which prevents protrusion of the weld bead into the interior of the pipe is the use of backplates or back-up shoes positioned on the interior of the pipe and over the gap between the pipe sections. The backplate prevents the weld bead from protruding into the interior of the pipe sections during the root pass of the weld bead. However, the use of backplates is also very time-consuming and is limited to relatively large diameter pipes. In addition, the backplate, in many instances, becomes welded to the interior of the pipe section during the laying of the root pass. Consequently, the backplate must later be removed which can be both costly and can adversely damage the weld bead during the removal of the welding plate.
A welding apparatus which overcomes many of the problems associated with past pipe welding methods is disclosed in U.S. Pat. No. 5,676,857. This patent discloses an improvement in the welding of the end of two pipe sections by the use of a welding apparatus having two welding bugs which continuously move on a track around the periphery of the pipe. The welding bugs include a special short circuiting power source to apply a root bead between the two ends of a pipe. The patent discloses that by using this system of welding with the proper bug speed and welding wire speed, only a slight burn through each edge of the bevel occurs and a small flat weld is formed on the interior of the pipe, thus eliminating the need to run an initial root pass from the interior of the pipe or to use other types of equipment inside the pipe such as a backplate. Although the method of welding disclosed in U.S. Pat. No. 5,676,857 overcomes several of the problems associated with the welding of pipe sections together, problems associated with the alloy composition of the weld metal of the root bead and the shielding of the weld metal of the root bead from the adverse affects of the atmosphere remain a problem.
The short circuiting power source is designed to be used with a solid wire electrode and, during welding, the weld metal is protected from the atmosphere by use of various types and blends of shielding gas. Although the use of a solid wire and shielding gas produces an excellent weld bead in many environments, the short circuiting welding method has several limitations when used to weld together the pipe sections. Because solid wire electrodes are used, the composition of the weld bead is limited to the available alloy compositions of electrodes for use in short circuiting welding. As a result, the composition of the weld bead may significantly deviate from the composition of the metal of the pipe sections. The composition of the weld metal should closely match the composition of the metal pipe to form a weld bead having optimum weld bead characteristics. Because the composition of the pipe will vary depending on the application of use for the pipe, problems will arise with the obtaining of a solid wire electrode which forms a weld metal that optimally bonds with the pipe sections.
Another limitation of the short circuiting welding process is that a shielding gas must be used to protect the weld bead from the adverse effects of the environment. The welding apparatus must include an arrangement for storing and directing shielding gas to the area of welding. Such an arrangement must include a mounting arrangement for the containers of shielding gas, regulators, flow meters, hoses, and other materials necessary to direct the shielding gas to the welding area during welding. The shielding gas prevents oxygen, nitrogen, hydrogen and other compounds in the atmosphere from reacting with the molten metal and/or being trapped in the molten metal. These elements and compound can cause porosity in the weld bead, cracking of the welding bead, spattering of the weld metal, etc., which can significantly compromise the strength and quality of the weld bead. The use of a shielding gas in a controlled indoor environment is effective in preventing the adverse effects on the weld bead from the environment; however, the use of shielding gases in an outdoor environment are highly susceptible to the effects of wind during the welding process. In order to minimize these effects, special shields must be erected around the perimeter of the electrode to shield the shielding gas from the wind during welding. The use of such shields or other welding configurations to minimize the effects of the atmosphere on the weld bead is both costly and significantly complicates the weld apparatus arrangement.
In view of the problems associated with welding of pipe sections in various types of environments and for producing a high quality weld bead having a composition substantially similar to the composition of the pipes that are being welded together, there is a need for an improved welding method and apparatus which can overcome such problems during the welding of pipe.